1. Field
The present invention relates generally to communications. More particularly, in aspects the invention relates to adaptive power control in wireless communication systems.
2. Background
Modern wireless communication systems are widely deployed to provide various types of communication applications, such as voice and data applications. These systems may be multiple access systems capable of supporting communication with multiple users by sharing the available system resources (e.g., spectrum and transmit power). Examples of multiple access systems include code division multiple access (CDMA) systems, time division multiple access (TDMA) systems, frequency division multiple access (FDMA) systems, time division duplexing (TDD) systems, frequency division duplexing (FDD) systems, 3rd generation partnership project long term evolution (3GPP LTE) systems, and orthogonal frequency division multiple access (OFDMA) systems. There are also point-to-point systems, peer-to-peer systems, and wireless local area networks (wireless LANs).
Generally, a wireless multiple access communication system can simultaneously support communications with multiple wireless terminals. Each terminal communicates with one or more base transceiver stations (BTSs or base stations) via transmissions on the forward and reverse links. The forward link or downlink refers to the communication link from a base transceiver station to a terminal, and the reverse link or uplink refers to the communication link from a terminal to a base transceiver station. Both forward and reverse communication links may be established via a single-in-single-out, multiple-in-single-out, single-in-multiple-out, or a multiple-in-multiple-out (MIMO) communication technique, depending on the number of transmitting and receiving antennae used for the particular link.
MIMO systems are of particular interest because of their relatively higher data rates, relatively longer coverage range, and relatively more reliable transmission of data. A MIMO system employs multiple (NT) transmit antennae and multiple (NR) receive antennae for data transmission. A MIMO channel formed by the NT transmit and NR receive antennae may be decomposed into NS independent channels, which are also referred to as spatial channels, where NS≦min {NT, NR}. Each of the NS independent channels corresponds to a dimension. The MIMO system can provide improved performance (e.g., higher throughput and/or greater reliability) if the additional dimensions created by the multiple transmit and receive antennae are used.
MIMO techniques may support both TDD and FDD systems. In a TDD system, the forward and reverse link transmissions are in the same frequency band, so that the reciprocity principle allows the estimation of the forward link channel from the reverse link channel, and vice versa. In an FDD system, different frequency bands may be used for forward and reverse link transmissions, making estimation of the reverse link channel from the forward link channel generally less reliable.
In many wireless communication systems, and particularly multiple access wireless communication systems, over-the-air power control is an important aspect of design. Because transmissions from one access terminal constitute interference for other terminals, it is desirable to keep the power of the transmissions to a value sufficient for reliable link communications, but not much higher. At the same time, access terminals are typically battery operated, so battery life may be an important performance characteristic. For this reason as well, it is desirable to limit the transmitted power and the associated battery power consumption to a level not much higher than that needed for reliable communications. This goal is often achieved through adaptive power control. Adaptive power control may be open loop, for example, where a wireless access terminal estimates a reverse link channel from an estimate of a forward link channel obtained from a pilot signal. Adaptive power control may also be close loop, for example, where an access terminal receives power control commands from a base transceiver station of a radio network. Exemplary method of power control in CDMA systems are described in U.S. Pat. No. 5,056,109; and in United States Patent Application Publication Number 2005/0197150. Both patent documents identified in the immediately preceding sentence are assigned to the assignee of the present invention and are incorporated by reference as if fully set forth herein, including figures, claims, and tables (if any).
Because access terminals are typically mobile, their operating environment is constantly changing in various respects. Thus, noise, interference, attenuation, distortion, fading, and other physical channel characteristics may be fast-changing, sometimes necessitating abrupt adjustments of the transmitted power. For systems that operate in a non-continuous manner over relatively short intervals, power adjustments may be made during breaks in transmissions. In certain TDMA systems, for example, the access terminal may transition from one transmit power level to another transmit power level during the time the access terminal is receiving and not transmitting. In other systems, such as certain FDD and OFDMA systems, power adjustments are made during continuous transmissions. These systems include Worldwide Interoperability for Microwave Access (WiMAX) systems, LTE systems, Ultra Mobile Broadband (UMB) systems, and IEEE 802.20 standard systems.
Relatively small power adjustments generally do not cause excessive spectral spread and its associated bandwidth utilization inefficiencies. Relatively large power adjustments, however, may cause excessive spectral spread and the accompanying bandwidth utilization inefficiencies.
A need thus exists in the art for apparatus, methods, and articles of manufacture that reduce spectral spread caused by large power fluctuations in power control settings.